Construction of a gigahertz-bandwidth radio-frequency scanning tunneling microscope based on a commercial low-temperature system

The highest frequency of the electric signal that a conventional scanning tunneling microscope (STM) can process typically lies in the kilohertz regime, imposing a limitation on its temporal resolution to the submillisecond regime. When extracting (feeding) the high frequency, or radio-frequency (RF), signal out of (into) the tunnel junction, the most challenging part is that the tunnel junction has a very high impedance, causing significant reflections. Here, we present a systematic solution on the construction of RF-STM with high sensitivity. To minimize radiation loss, using coaxial cables as conducting wires, we designed an active impedance matching network (IMN) based on a field-effect transistor, which can provide impedance matching over a wide frequency range and can bridge the enormous impedance difference associated with the tunnel junction. To shorten the signal cable before amplification, the STM probe itself was directly mounted on the IMN as the input pin, which is an unprecedented attempt to minimize the undesired parasitic capacitances. Furthermore, we employed a two-stage cryogenic SiGe low noise amplifier and a high-end spectrum analyzer to amplify and subsequently analyze the RF signal of interest. After this systematic engineering, the bandwidth of our STM has been improved to the gigahertz regime, implying a six orders of magnitude improvement. The sensitivity level of our newly built RF-STM is measured to be better than 1.0 pA/√Hz at 200 MHz at 78 K. The RF-STM also finds its application in nanoscale thermometry. Our efforts in its instrumentation should contribute to the development of high frequency scanning tunneling microscopy.The highest frequency of the electric signal that a conventional scanning tunneling microscope (STM) can process typically lies in the kilohertz regime, imposing a limitation on its temporal resolution to the submillisecond regime. When extracting (feeding) the high frequency, or radio-frequency (RF), signal out of (into) the tunnel junction, the most challenging part is that the tunnel junction has a very high impedance, causing significant reflections. Here, we present a systematic solution on the construction of RF-STM with high sensitivity. To minimize radiation loss, using coaxial cables as conducting wires, we designed an active impedance matching network (IMN) based on a field-effect transistor, which can provide impedance matching over a wide frequency range and can bridge the enormous impedance difference associated with the tunnel junction. To shorten the signal cable before amplification, the STM probe itself was directly mounted on the IMN as the input pin, which is an unprecedented attempt to...